1. Field of the Invention
The present invention relates to a lubricating oil composition and more particularly to a lubricating oil composition which is excellent in biodegradability, high-temperature cleanliness and anti-seizure performance and is therefore conductive to environmental protection.
2. Prior Art
Two-cycle engines raise problems as to possible environmental pollution since they discharge an unburned engine oil as a by-product of their exhaust gases due to their lubrication mechanism. For example, outboard engines used in rivers, lakes or oceans discharge exhaust gases containing unburned-oil into the water so that they raise problems as to water pollution due to the unburned oil. Chain saws and other forestry machinery may also cause the pollution of forests and rivers due to the unburned oil.
In Europe, there have been substantial activities in attempts to establish laws, regulations and standards concerning the biodegradability of the two-cycle engine oils from the standpoint of environmental protection. Because of this, a two-cycle engine oil having biodegradabillty has heretofore been considered to be developed and it has already been sold from several oil makers. These oils, in all cases, comprise, as a specific base oil an ester compound (based on a mixture of saturated and unsaturated fatty acid esters each having 16-18 carbon atoms) and a mineral oil-based solvent. They contain an amino amide type ashless dispersant as an additive.
These commercially available oils already passed the TC-WII which is a performance standard of oils for outboard engines. However, when these oils are used in recently manufactured high-performance water-cooled outboard engines and air-cooled engines, they are considered to raise problems as to ring sticking and piston seizure due to their insufficient thermal stability as a lubricating oil.
Because of this, a biodegradable lubricating oil having better thermal stability and more excellent lubricity has been sought.